1. Field of the Invention
The present invention relates to a backlight inverter to drive a light source of a backlight device for a liquid crystal display, and particularly to a backlight inverter to light a plurality of lamps for a large liquid crystal television, and also to a method of driving the backlight inverter.
2. Description of the Related Art
Recently, a liquid crystal display (hereinafter referred to as LCD) is extensively used as a display device for a personal computer, and the like. The LCD requires a lighting system such as a backlight for illuminating its screen. In order to illuminate such a LCD screen brightly, a plurality of cold cathode fluorescent lamps (hereinafter referred to as CCFL) are used as the light source and are discharged and lit simultaneously.
Generally, at the time of starting discharging a CCFL, a high frequency voltage of about 60 kHz and 1600 V is to be generated out of a DC input voltage of about 12 V at the secondary side of an inverter transformer, and therefore an inverter circuit is employed which includes an inverter unit incorporating a full bridge circuit or a Royer circuit and adapted to drive a backlight. Once the CCFL discharge starts, such an inverter circuit operates to step the voltage at the secondary side of the inverter transformer down to about 600 V which is required for keeping the CCFL discharging. Usually, this voltage control operation is performed by pulse width modulation (PWM).
In some conventional backlight inverter circuits, a resonant circuit is formed of a leakage inductance at the secondary side of a transformer and a parasitic capacitance of a discharge lamp connected as a load, and the primary side of the transformer is driven at the resonance frequency of the resonant circuit (refer to, for example, Patent Document 1).
This drive by the resonance frequency, however, involves a phase difference between voltage and current at the primary side of the transformer and does not necessarily provide good power efficiency. Also, a resonance frequency of a high order is found at the secondary side of the transformer, and it possibly happens that the transformer operates at such a resonance frequency or operates under the influence of such a resonance frequency, thus complicating the transformer designing. And, a CCFL for a backlight undergoes substantial fluctuations of lamp impedance depending on temperature and lamp current, especially immediately after cold starting. The large fluctuation of lamp impedance causes fluctuation in lamp voltage, and consequently the parasitic capacitance formed at the lamp is caused to fluctuate, too.
In order to deal with the difficulties described above, a backlight inverter disclosed in Patent Document 2 is structured with a transformer including a resonant circuit formed of a parasitic capacitance of a discharge lamp, and an H-bridge circuit to drive the primary side of the transformer at a frequency which is lower than the resonance frequency of the resonant circuit and also at which the voltage-current phase difference θ at the primary side of the transformer is kept within a predetermine range from the minimum point. This enhances the power efficiency of the transformer and also makes the transformer less susceptible to the influences of the frequency of a high order, thus facilitating the transformer designing.
Patent Document 3 discloses a method of driving an inverter, which stabilizes the oscillating operation thereby preventing lamps from flickering and circuit elements from generating noises. This method drives an inverter provided with a step-up transformer which operates such that a DC current is applied to the input winding, the current applied is turned on and off by a switch element, and an alternate voltage is outputted from the output winding, wherein the inverter is driven at a frequency staying out of the frequency range where the input-output voltage phase difference of the step-up transformer is between 50 to 130 degrees. Consequently, the turn number of windings, the air gap, and the degree of coupling are adjusted thus deteriorating the power efficiency, but the fluctuation of the output voltage due to the fluctuation of load impedance is reduced thus stabilizing the oscillation.
Since the driving method described above is adapted for lighting one CCFL or a couple of CCFLs, it is difficult for one backlight inverter to stably light more CCFLs, for example eight to sixteen CCFLs, and the lamp voltages of the individual CCFLs fluctuate causing fluctuation of the currents flowing in the parasitic capacitances of the CCFLs, which makes the brightness unstable causing the screen image of the LCD to flicker.
Also, in a backlight for a large television, a plurality of CCFLs are disposed behind the LCD, and in order to achieve a low cost backlight inverter, the plurality of CCFLs are lit such that a plurality of field effect transistor (FET) bridges, each of which is connected to a plurality of inverter transformers, are driven by one control IC. In such a structure, the CCFL undergoes a substantial fluctuation according to lamp current, especially immediately after cold starting. The substantial fluctuation of lamp impedance leads to fluctuation of lamp voltage, and consequently current flowing in the parasitic capacitance of the CCFL is caused to fluctuate. Thus, at the time of cold starting, even if the control IC of the backlight inverter functions to control the lamp current, the current flowing in the parasitic capacitance is caused to fluctuate due to the fluctuation of the lamp voltage. As a result, the lamp current flowing in the CCFL is not stabilized, and the brightness becomes instable causing the screen image of the LCD to flicker. If the lamp temperature becomes stabilized, then the lamp current is also stabilized thus easing and eliminating the flickering image, but it takes several minutes after start-up before the stabilization is reached, and the stabilization must be reached more rapidly. This flickering problem may be simply solved if the lamp current is controlled by providing one control IC for each of the CCFLs, but this solution approach pushes up production cost of the backlight inverter significantly.    Patent Document 1: U.S. Pat. No. 6,114,814    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-168585    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-201457